As of 2012, the FDA (the American Food & Drugs Administration) stated that anything up to 400mg of caffeine a day for healthy adults* is agreeable - and wasn’t associated with any adverse health effects. However, for pregnant women, the NHS recommends that you don’t exceed 200mg a day - although, interestingly, the medical evidence behind this remains equivocal.
In day to day terms, that's roughly the amount of caffeine in four cups of brewed coffee, 6/7 coffee pods, or 10 cans of coke.
*Although caffeine use may be safe for adults, heavy caffeine use can cause unpleasant side effects & may not be a good choice for people who are taking certain medication or pregnant. If you find you're having any adverse side effects to caffeine, please get in contact with your local doctor.